Chapter 12: Mendel & Inheritance
This chapter delves into Gregor Mendel's groundbreaking experiments with pea plants, which established the foundational principles of inheritance through the concepts of dominant and recessive traits.
Review Questions:
Q1: Mendel performed hybridizations by transferring pollen from the _______ of the male plant to the female ova.
A: a. antherQ2: Which is one of the seven characteristics that Mendel observed in pea plants?
A: b. seed textureQ3: What F1 offspring would you expect from crossing green-seeded and yellow-seeded true-breeding parents (yellow dominant)?
A: b. 100 percent yellow seedsQ4: If 650 inflated-pod plants are found in the F2 generation, how many constricted-pod plants would you expect (3:1 ratio)?
A: b. 165Q5: What ratio of offspring would result from a cross between a white-eyed male and a heterozygous red-eyed female?
A: 1 red-eyed female : 1 white-eyed female : 1 red-eyed male : 1 white-eyed male (i.e., 1:1:1:1)Q6: What are the possible genotypes and phenotypes for a cross between PpYY and ppYy?
A: Genotypes: PpYy, PpYY, ppYy, ppYY
Phenotypes: Purple/yellow and white/yellow
Punnett square size: 4 squares (2 × 2)Q7: A true-breeding violet terminal × white axial flower cross — what is expected in the F2?
A: d. 75% violet flowers in an axial positionQ8: The observable traits expressed by an organism are described as its ________.
A: a. phenotypeQ9: A recessive trait will be observed in individuals that are ________ for that trait.
A: c. homozygousQ10: If black and white mice mate and all offspring are gray, what inheritance pattern is this?
A: d. incomplete dominanceQ11: ABO blood groups are an example of:
A: d. multiple alleles and codominanceQ12: In a mating between two heterozygous carriers of a recessive lethal allele, what genotypic ratio is expected?
A: c. 1:2:0 (homozygous recessives die)Q13: If the allele encoding polydactyly is dominant, why do most people have five fingers?
A: d. The polydactyl allele is very rare in the human population.Q14: A farmer breeds speckled chickens from black and white parents. What offspring will appear after breeding speckled x speckled?
A: c. 50% speckled, 25% black, 25% whiteQ15: In a dihybrid cross (AaBb), what is the expected gamete ratio for F1 (AB, Ab, aB, ab)?
A: a. 1:1:1:1Q16: The forked line and probability methods use which probability rule?
A: b. product ruleQ17: How many different genotypes and phenotypes are expected in a trihybrid cross (all heterozygous parents)?
A: d. 27 genotypes; 8 phenotypesQ18: Labrador retriever fur color (controlled by E and B alleles) is an example of what inheritance pattern?
A: a. epistasisQ19: Which scenario does not follow the Law of Independent Assortment?
A: d. Men are more likely to experience hemophilia than women.:
🌱 Mendel and the Foundations of Genetics
🧪 Why Mendel’s Work Was Revolutionary
Selected pea plants for their distinct traits and ability to self- or cross-fertilize.
Used true-breeding strains and quantitative methods over large sample sizes.
Introduced the concept of discrete units of heredity (genes), which contrasted the blending theory.
🌼 Mendel’s Experimental Method
Three Stages:
True-breeding strains with clear traits (P generation)
Cross-fertilization → F1 generation (all show dominant trait)
Self-fertilization of F1 → F2 generation (3:1 phenotypic ratio)
Key Results:
F1: all dominant trait
F2: 3 dominant : 1 recessive → Actually 1:2:1 genotype (homo dom : hetero : homo rec)
🧬 Mendel’s Principles
1. Law of Segregation
Alleles separate during gamete formation (meiosis).
Each gamete receives one allele.
2. Law of Independent Assortment
Alleles for different genes assort independently during gamete formation.
Explains dihybrid 9:3:3:1 ratio.
📊 Probability in Genetics
Product Rule (AND): Multiply probabilities of independent events.
Sum Rule (OR): Add probabilities of mutually exclusive events.
🧪 Test Cross
Cross a dominant phenotype (unknown genotype) with homozygous recessive.
Resulting offspring ratios reveal genotype.
🧾 Vocabulary Recap
Allele: Alternative version of a gene
Homozygous / Heterozygous
Genotype: Genetic makeup
Phenotype: Physical expression
🧬 Extensions to Mendelian Genetics
🔸 Incomplete Dominance
Heterozygote shows intermediate phenotype (e.g., red + white = pink)
🔸 Codominance
Both alleles expressed (e.g., AB blood type)
🔸 Multiple Alleles
More than two alleles exist in population (e.g., IA, IB, i for blood types)
🔸 Polygenic Inheritance
Trait controlled by multiple genes (e.g., height, skin color)
🔸 Pleiotropy
One gene influences multiple traits (e.g., sickle cell, cystic fibrosis)
🔸 Environmental Influence
Trait expression depends on external factors (e.g., temperature-sensitive pigment in Siamese cats)
🔸 Epistasis
One gene masks the expression of another (e.g., albinism gene masking coat color)
🧬 Dihybrid Cross (Two Traits)
Classic 9:3:3:1 phenotypic ratio (e.g., RrYy x RrYy)
Demonstrates independent assortment
🔍 Human Traits and Pedigree Analysis
Dominant disorders: Appear every generation (e.g., Huntington's)
Recessive disorders: May skip generations (e.g., albinism)
Use pedigrees to trace inheritance in families.
♀♂ Sex-linked Traits (Mostly X-linked)
Males more affected (only one X)
Females can be carriers
Examples: Color blindness, hemophilia
⚠ Lethal Alleles
Recessive lethal: Only lethal in homozygous state (2:1 ratio in offspring)
Dominant lethal: Lethal even in heterozygous state (e.g., Huntington’s)
What genotype ratio do you expect from a monohybrid cross of two heterozygotes (Aa x Aa)?
✎ Answer: 1 AA : 2 Aa : 1 aaWhat is the probability of an offspring being homozygous recessive from an Aa x Aa cross?
✎ Answer: 1/4What’s the probability of producing a plant with yellow AND round seeds in a dihybrid cross (YyRr x YyRr)?
✎ Answer: 9/16 (3/4 yellow × 3/4 round)If two carriers of a recessive lethal allele mate, what is the expected genotypic ratio among surviving offspring?
✎ Answer: 2:1 (heterozygous : homozygous dominant)How does the product rule apply in a trihybrid cross when calculating the probability of a specific genotype like aabbcc?
✎ Answer: Multiply the chance of each independent event → (1/4) × (1/4) × (1/4) = 1/64
🧬 Non-Mendelian Inheritance
What is codominance? Give an example.
✎ Answer: Both alleles are fully expressed in the heterozygote; example: AB blood typeWhat is the phenotype ratio for incomplete dominance when two heterozygotes are crossed (e.g., pink flowers)?
✎ Answer: 1 red : 2 pink : 1 whiteWhat pattern of inheritance is observed when one gene masks another, such as pigment production overriding coat color in mice?
✎ Answer: EpistasisHow many phenotypes are expected in the F2 generation of a dihybrid cross?
✎ Answer: 4 phenotypes (9:3:3:1 ratio)
🧬 X-linked Traits & Sex-linked Inheritance
Why are X-linked recessive traits more common in males?
✎ Answer: Males have only one X chromosome, so a single recessive allele will be expressedIf a carrier female for color blindness (XᴺXⁿ) has children with a normal male (XᴺY), what is the probability a son will be colorblind?
✎ Answer: 50%What is hemizygous, and why are males considered hemizygous for X-linked traits?
✎ Answer: Hemizygous = having only one allele for a gene; males have only one X chromosome
👨👩👧👦 Pedigree Analysis
In a pedigree, if two unaffected parents have an affected child, what is the most likely inheritance pattern?
✎ Answer: Autosomal recessiveIf a trait appears in every generation and affects both males and females equally, what pattern is this?
✎ Answer: Autosomal dominantIf a trait only affects males and is passed from carrier mothers, what pattern is this?
✎ Answer: X-linked recessive